Abstract:

The diffractive optical element is constituted by a glass material and a
resin material whose dn/dT representing refractive index variation with
temperature is larger than that of the glass material, the resin material
being in close contact with or facing the glass material, and a
diffractive grating formed at a close contact portion or a facing portion
between the glass material and the resin material. The resin material is
a mixture material of (a) a resin base material, (b) first fine particles
formed of a first material whose dn/dT is equal to or higher than
-1×10-5(/° C.) and (c) second fine particles formed of
a second material whose Abbe constant is lower than that of the glass
material. The diffractive optical element can maintain high diffraction
efficiency in a wide wavelength region even if temperature changes,
without generating unnecessary diffracted light.

Claims:

1. A diffractive optical element comprising:a glass material;a resin
material whose dn/dT representing a refractive index variation with
temperature is larger than that of the glass material, the resin material
being in close contact with or facing the glass material; anda
diffractive grating formed at a close contact portion or a facing portion
between the glass material and the resin material,wherein the resin
material is a mixture material of (a) a resin base material, (b) first
fine particles formed of a first material whose dn/dT satisfies a
condition of dn/dT≧-1.times.10.sup.-5(/° C.) and (c) second
fine particles formed of a second material whose Abbe constant is lower
than that of the glass material.

2. A diffractive optical element according to claim 1, wherein at least
one of the first fine particles and the second fine particles are
particles whose particle diameter is equal to or smaller than 100 nm.

3. A diffractive optical element according to claim 1, wherein a mixing
ratio in volume of the first fine particles to the resin base material is
equal to or higher than 20%.

4. A diffractive optical element according to claim 1, wherein the resin
material has a lower refractive index and a lower Abbe constant than
those of the glass material.

5. A diffractive optical element according to claim 1, wherein the glass
material is a low melting point glass whose glass transition temperature
is equal to or lower than 600.degree. C.

6. An optical system comprising:a diffractive optical element,wherein the
diffractive optical element comprising:a glass material;a resin material
whose dn/dT representing a refractive index variation with temperature is
larger than that of the glass material, the resin material being in close
contact with or facing the glass material; anda diffractive grating
formed at a close contact portion or a facing portion between the glass
material and the resin material,wherein the resin material is a mixture
material of (a) a resin base material, (b) first fine particles formed of
a first material whose dn/dT satisfies a condition of
dn/dT≧-1.times.10.sup.-5(/° C.) and (c) second fine
particles formed of a second material whose Abbe constant is lower than
that of the glass material.

7. An optical apparatus comprising:an optical system including a
diffractive optical element,wherein the diffractive optical element
comprising:a glass material;a resin material whose dn/dT representing a
refractive index variation with temperature is larger than that of the
glass material, the resin material being in close contact with or facing
the glass material; anda diffractive grating formed at a close contact
portion or a facing portion between the glass material and the resin
material,wherein the resin material is a mixture material of (a) a resin
base material, (b) first fine particles formed of a first material whose
dn/dT satisfies a condition of dn/dT≧-1.times.10.sup.-5(/°
C.) and (c) second fine particles formed of a second material whose Abbe
constant is lower than that of the glass material.

Description:

BACKGROUND OF THE INVENTION

[0001]The present invention relates to a diffractive optical element used
for an optical system such as an image taking optical system.

[0002]Methods for reducing chromatic aberrations of an optical system are
known which provide a diffractive optical element constituting part of
the optical system, the methods being disclosed in "SPIE Vol. 1354
International Lens Design Conference (1990)", Japanese Patent Laid-Open
Nos. 04-213421 and 06-324262, and U.S. Pat. No. 5,044,706.

[0003]Such a diffractive optical element has a shape in which a phase term
defined by an optical path difference function is added to a base shape.
The base shape is, for example, a shape of a surface of a lens
constituting an optical system, the shape of the lens surface being a
spherical shape, an aspheric shape or a flat surface shape. Moreover, an
additional amount of an optical path length due to a structure in which a
diffractive grating shape is added to the lens surface is expressed by
using an optical path difference function φ(h) defined by
φ(h)=(C1h2+C2h4+C3h.sup.6+ . . . )×2π/λ
where h represents a height from an optical axis, Pn represents an
optical path difference function coefficient of an n-th order (even
number order), and λ represents a wavelength.

[0004]For example, when a diffractive grating shape (plural orbicular
zones) formed according to the optical path difference function φ(h)
is concentrically added to a lens surface whose curvature is R, employing
a diffractive grating shape satisfying the following expression makes it
possible to produce a diffractive lens having a diffraction effect.

x = R - R 2 - h 2 + ( k - Φ ( h ) 2 π
) × d ##EQU00001##

[0005]where x represents a position in an optical axis direction, k
represents a number of the orbicular zone counted from a center, and d
represents a grating thickness.

[0006]In the above expression, the first and second terms indicate the
base shape and the third term indicates the phase term defined by the
optical path difference function to be added to the base shape. Regarding
the second term, the position x becomes discontinuous at portions where
the orbicular zone numbers change, which generates a grating shape.

[0007]When the diffractive optical element is used in the optical system,
it is necessary that diffraction efficiency for a designed diffraction
order be sufficiently high in the entire wavelength region of light
entering the optical system (hereinafter, the wavelength region of the
light is referred to as "use wavelength region"). If the diffraction
efficiency for the designed diffraction order is low, a lot of light rays
of diffraction orders other than the designed diffraction order exist,
and reach other positions where the designed diffraction order light rays
do not reach to generate flare.

[0008]FIG. 13 shows an example of a conventional diffractive optical
element. In a left part of FIG. 13, reference numeral 122 denotes
orbicular zones of a diffractive grating of the diffractive optical
element. Changing pitches between the gratings makes it possible to
provide an optical power. Moreover, in a right part of FIG. 13, a first
diffractive grating 135 and a second diffractive grating 136 are disposed
so as to face each other with an air layer 133 therebetween. Such a
configuration of the diffractive optical element can provide high
diffraction efficiency in a wide wavelength region.

[0009]FIG. 14 shows another example of a conventional diffractive optical
element (multilayer diffractive optical element). Reference numeral 141
denotes a first diffractive grating, reference numeral 142 denotes a
second diffractive grating, and reference numeral 143 denotes an air
layer. The first diffractive grating 141 and the second diffractive
grating 142 are formed of materials having mutually different dispersions
(Abbe constants νd). For example, the first diffractive grating 141 is
formed of a first ultraviolet curable resin (nd=1.635, νd=23.0), and
the second diffractive grating 142 is formed of a second ultraviolet
curable resin (nd=1.524, νd=50.8). A grating thickness d1 of the first
diffractive grating 141 is 7.8 μm, and a grating thickness d2 of the
second diffractive grating 142 is 10.7 μm. A thickness d3 of the air
layer 143 is 1.0 μm. A grating pitch is 140 μm, and a designed
diffraction order is +1. The diffraction efficiency in this case is
approximately 100% for light 144 in the entire visible wavelength region.

[0010]FIG. 17 shows a still another example of a conventional diffractive
optical element (contact type diffractive optical element). This
diffractive optical element has a structure in which a first diffractive
grating 51 and a second diffractive grating 52 are in close contact with
each other, without having the air layer formed in the multilayer
diffractive optical element shown in FIG. 14. In this diffractive optical
element, the first diffractive grating 51 is formed of a material having
a refractive index lower than that of the second diffractive grating 52.
Such a contact type diffractive optical element can be produced by, for
example, molding one of the first and second diffractive gratings 51 and
52 by using a glass material and then placing an uncured resin material
on the grating of the glass material to cure it.

[0011]When n(λ) represents a refractive index of the first
diffractive grating 51 formed of a low refractive index material for a
wavelength λ, and n' (λ) represents a refractive index of the
second diffractive grating 52 formed of a high refractive index material
for the wavelength λ, a phase deviation is expressed by the
following expression:

φ(λ)={n'(λ)-n(λ)}d/λ

[0012]When m represents a diffraction order, diffraction efficiency
η(λ) is expressed by the following expression:

[0013]One of advantages obtained by using glass and resin materials is
that a number of selectable materials is increased. Optical glass
materials include many glass materials having various combinations of
refractive indexes and Abbe constants. On the other hand, optical resin
materials include a small number of resin materials, and therefore
selectable materials are limited. Moreover, the resin materials generally
have lower refractive indexes than those of the glass materials.

[0014]It is preferable for producing the multilayer diffractive optical
element to combine a material having a low refractive index and a high
dispersion and a material having a high refractive index and a low
dispersion. Therefore, using one of many types of glass materials enables
production of a diffractive grating having higher diffraction efficiency.

[0015]Moreover, the above-described method which molds the diffractive
grating by using the glass material and places the uncured resin material
thereon to cure it is preferable because the glass material is more
resistant to ultraviolet light and heat as compared with the resin
material, and the grating thickness and the grating shape of the glass
material are not changed when the resin material is cured.

[0016]Additionally, Japanese Patent Laid-Open No. 2005-338798 discloses
that a material in which fine particles are dispersed is used for
temperature compensation in a diffractive grating formed by combining
resin materials. Moreover, Japanese Patent Laid-Open No. 2005-38481
discloses that producing a single-layer diffractive optical element by
using an a thermal material in which inorganic particles are dispersed in
a resin material improves its temperature property.

[0017]However, while a refractive index variation with temperature (dn/dT)
of the resin material is a negative value of
-1.0×10-4(/° C.), dn/dT of the glass material is about
1/10 of that of the resin material and is generally a positive value.

[0018]Further, in the diffractive optical element in which the first resin
material and the second resin material are combined, a temperature
variation of the diffractive optical element due to environmental
temperature change or the like generates a phase deviation with respect
to a designed additive phase amount since the refractive index variations
of the first resin material and the second resin material are different
from each other. That is,

φ(λ)={n'(λ)+δn'(λ)-n(λ)-δn(λ-
)}d/λ

is established, and therefore the diffraction efficiency η is
deteriorated by that phase deviation.

[0019]In particular, when the diffractive optical element is used for an
image taking optical system (image forming optical system), concentric
flare light is generated around a high-intensity light source included in
a scene for image taking.

[0020]Furthermore, Japanese Patent Laid-open No. 2005-338798 does not
consider that the fine particles are mixed in the resin material in order
to bring dn/dT of the resin material close to dn/dT of a glass material,
and only fine-tunes the refractive index and the Abbe constant of the
resin material. This is because mixing the fine particles into the resin
material for bringing dn/dT of the resin material close to dn/dT of the
glass material provides to the resin material a refractive index which
makes it difficult to design a diffractive grating capable of obtaining
high diffraction efficiency in a wide wavelength region.

[0021]Moreover, Japanese Laid-Open No. 2005-38481 discloses a case where
the single-layer diffraction optical element is used for a single
wavelength such as a laser. Therefore, it is not necessary to improve
diffraction efficiency in a wide wavelength region by combining two or
more materials. Accordingly, there is no limitation on the refractive
index and the Abbe constant of the use material, and it is not necessary
to mix two or more materials at all.

SUMMARY OF THE INVENTION

[0022]The present invention provides a diffractive optical element capable
of maintaining high diffraction efficiency in a wide wavelength region
even if temperature changes, without generating unnecessary diffracted
light, and provides an optical system and an optical apparatus using the
same.

[0023]The present invention provides as one aspect thereof a diffractive
optical element including a glass material, a resin material whose dn/dT
representing a refractive index variation with temperature is larger than
that of the glass material, the resin material being in close contact
with or facing the glass material, and a diffractive grating formed at a
close contact portion or a facing portion between the glass material and
the resin material. The resin material is a mixture material of a resin
base material, first fine particles formed of a first material whose
dn/dT satisfies a condition of dn/dT≧-1×10-5(/°
C.) and second fine particles formed of a second material whose Abbe
constant is lower than that of the glass material.

[0024]The present invention provides as other aspects thereof an optical
system including the above-described diffractive optical element, and an
optical apparatus including the optical system.

[0025]Other aspects of the present invention will become apparent from the
following description and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a cross sectional view of an image taking optical system
using a diffractive optical element of Embodiment 1 of the present
invention.

[0027]FIG. 2 is a cross sectional view and a partially enlarged view of
the diffractive optical element of Embodiment 1.

[0028]FIG. 3 shows designed diffraction efficiency for first order
diffracted light of a diffractive optical element which is a comparative
example with respect to Embodiment 1.

[0029]FIG. 4 shows diffraction efficiency for the first order diffracted
light of the diffractive optical element of the comparative example when
temperature thereof rises by 20° C.

[0030]FIG. 5 shows designed diffraction efficiency for zeroth order
diffracted light and designed diffraction efficiency for second order
diffracted light of the diffractive optical element of the comparative
example.

[0031]FIG. 6 shows diffraction efficiency for the zeroth order diffracted
light and diffraction efficiency for the second order diffracted light of
the diffractive optical element of the comparative example when the
temperature thereof rises by 20° C.

[0032]FIG. 7 shows designed diffraction efficiency for first order
diffracted light of the diffractive optical element of Embodiment 1.

[0033]FIG. 8 shows designed diffraction efficiency for zeroth order
diffracted light and designed diffraction efficiency for second order
diffracted light of the diffractive optical element of Embodiment 1.

[0034]FIG. 9 shows diffraction efficiency for the first order diffracted
light of the diffractive optical element of Embodiment 1 when temperature
thereof rises by 20° C.

[0035]FIG. 10 shows diffraction efficiency for the zeroth order diffracted
light and diffraction efficiency for the second order diffracted light of
the diffractive optical element of Embodiment 1 when the temperature
thereof rises by 20° C.

[0036]FIG. 11 shows diffraction efficiency for the first order diffracted
light of the diffractive optical element of Embodiment 1 when a mixing
ratio of SiO2 in the element is 50% and the temperature of the
element rises by 20° C.

[0037]FIG. 12 shows diffraction efficiency for the zeroth order diffracted
light and diffraction efficiency for the second order diffracted light of
the diffractive optical element of Embodiment 1 when the mixing ratio of
SiO2 in the element is 50% and the temperature of the element rises
by 20° C.

[0038]FIG. 13 is an explanatory drawing of a conventional multilayer
diffractive optical element.

[0039]FIG. 14 is an explanatory drawing of another conventional multilayer
diffractive optical element.

[0040]FIG. 15 shows designed diffraction efficiency for first order
diffracted light of a diffractive optical element of Embodiment 2 of the
present invention.

[0041]FIG. 16 shows designed diffraction efficiency for zeroth order
diffracted light and designed diffraction efficiency for second order
diffracted light of the diffractive optical element of Embodiment 2.

[0043]Exemplary embodiments of the present invention will hereinafter be
described with reference to the accompanying drawings.

Embodiment 1

[0044]FIG. 1 shows an example of an image taking optical system whose
focal length is 400 mm which includes a diffractive optical element of a
first embodiment (Embodiment 1) of the present invention. In FIG. 1,
reference numeral 10 denotes an image pickup apparatus (optical
apparatus) including an image taking optical system 11. The image taking
optical system 11 may be detachable as an interchangeable lens (optical
apparatus) to a main body of the image pickup apparatus provided with an
image pickup element described later.

[0045]The image taking optical system 11 has plural lens units from an
object side to an image side. Reference numeral 1 denotes the diffractive
optical element provided in a first lens unit disposed closest to an
object. Reference numeral 2 denotes an aperture stop. Reference numeral 3
denotes the image pickup element such as a CCD sensor or a CMOS sensor
which is disposed on an image plane of the image taking optical system
11. Reference numeral 4 denotes a light flux entering the image taking
optical system 11 and forming a maximum angle of view. Reference numeral
5 denotes an optical axis of the image taking optical system 11.

[0046]In this embodiment, the diffractive optical element 1 is disposed
between a first (object side) lens element and a second (image side) lens
element that constitute the first lens unit, which makes it easy for
light from a high-intensity light source such as the sun to enter the
diffractive optical element 1 from an outside of an effective angle of
view. As a countermeasure thereagainst, reducing a number of grating
walls of a diffractive grating of the diffractive optical element 1 as
much as possible, that is, lowering a grating height is effective.
Therefore, in the diffractive optical element 1, combining a glass
material and a resin material advantageously increases a number of
selectable materials as compared with a case where the diffractive
grating is provided between resin materials.

[0047]On the other hand, in order to use a diffractive optical element for
an image taking optical system, it is necessary to suppress a phenomenon
of generation of unnecessary diffracted light around a high-intensity
light source as much as possible. Therefore, it is important to increase
diffraction efficiency, and deterioration of the diffraction efficiency
due to variation of a refractive index of the material of the diffractive
optical element with environmental temperature change is not permitted.

[0048]In the diffractive optical element 1 of this embodiment, a glass
material and a resin material whose dn/dT representing a refractive index
variation with temperature is larger than that of the glass material are
brought into close contact with each other, and the diffractive grating
is provided at a close contact portion between the glass material and the
resin material.

[0049]Next, description will be made of a diffractive optical element as a
comparative example with respect to the diffractive optical element 1 of
this embodiment. As a glass material, for example, K-VC79 made by Sumita
Optical Glass, Inc. is used. As a resin material, a mixture material of a
resin base material and fine particles is used. As the resin base
material, for example, ultraviolet (UV) curable resin RC1-C001 made by
Dainippon Ink And Chemicals (DIC), Inc. is used, and as a material of the
fine particles, ITO (Indium Tin Oxide) is used. The ITO fine particles
are dispersed (mixed) in the resin base material at a volume ratio of
12.0% with respect to the resin base material.

[0050]K-VC79 has a refractive index nd of 1.61038 and an Abbe constant
νd of 57.93. The mixture resin material of RC1-C001 and the ITO fine
particles (12.0%) has a refractive index nd of 1.5638 and an Abbe
constant νd of 23.22.

[0051]Setting a grating height to 12.6 μm by using these materials
provides good diffraction efficiency for first order diffracted light in
a wide wavelength range from 400 nm (or about 450 nm) to 700 nm as shown
in FIG. 3. FIG. 3 shows diffraction efficiency at a room temperature or a
designed temperature (hereinafter referred to as "standard temperature").
The refractive index variation with temperature (hereinafter referred to
as "temperature refractive index variation") dn/dT of the resin material
is -1.2×10-4(/° C.). On the other hand, dn/dT of K-VC79
that is the glass material is approximately 6.0×10-6(/°
C.). In other words, the temperature refractive index variation of the
resin material is extremely larger than that of the glass material.

[0052]FIG. 4 shows diffraction efficiency of the first order diffracted
light in a case where the temperature of the diffractive optical element
of the comparative example rises from the standard temperature by
20° C., and thereby refractive indexes of the respective materials
are changed. As shown in FIG. 4, the diffraction efficiency at about 500
nm is especially deteriorated to approximately 98%, which is low
diffraction efficiency that cannot be ignored in a normal image taking
optical system.

[0053]Moreover, FIGS. 5 and 6 show changes of diffraction efficiency for
zeroth order diffracted light and diffraction efficiency for second order
diffracted light with temperature change in the diffractive optical
element of the comparative example. FIG. 5 shows the diffraction
efficiency for the zeroth order diffracted light and the diffraction
efficiency for the second order diffracted light in a case where each
material has a designed refractive index (at the standard temperature).
In a wavelength region from about 450 nm to about 700 nm, the diffraction
efficiency for each order diffracted light is approximately 0.1%, which
is very low.

[0054]On the other hand, FIG. 6 shows the diffraction efficiency for the
zeroth order diffracted light and the diffraction efficiency for the
second order diffracted light when the temperature of the diffractive
optical element of the comparative example rises from the standard
temperature by 20° C. At a wavelength of about 500 nm, the
diffraction efficiency for each order diffracted light is deteriorated to
approximately 0.5% or a level exceeding 0.6%. Such deterioration of the
diffraction efficiency of each of the zeroth order diffracted light and
the second order diffracted light causes, when image pickup is performed
for a scene including a high-intensity light source, concentric flare
around the light source, which deteriorates a captured image.

[0055]FIG. 2 shows the diffractive optical element 1 of this embodiment in
detail. The diffractive optical element 1 has a structure in which a
diffractive grating is disposed between a first lens element 22 and a
second lens element 23, the diffractive grating being formed of an
ultraviolet curable resin material 26 whose refractive index for a d-line
is nd1 and a glass mold material whose refractive index for the d-line is
nd2. The relationship between the refractive indexes is nd1<nd2. The
first lens element 22 and the second lens element 23 also serve as
substrates for the diffractive optical element.

[0056]FIG. 2 includes an enlarged schematic view of part of the
ultraviolet curable resin material (hereinafter referred to as "UV
curable resin") 26. The UV curable resin 26 is produced by dispersing
(mixing) fine particles (first fine particles) 31 made of silica (silicon
dioxide, SiO2) that is a first material in the RC1-C001 that is the
above-described resin base material 30 at a volume ratio of 20% with
respect to the resin base material 30.

[0057]Although the first material is not limited to silica, it is
necessary that the first material satisfy the following condition:

dn/dT≧-1×10-5(/° C.)

(that is, dn/dT is equal to or higher than -1×10-5(/°
C.))

[0058]Further, it is preferable that a particle diameter (average particle
diameter) of the silica fine particles 31 be equal to or smaller than 100
nm. It is more preferable that the particle diameter thereof be equal to
or smaller than 50 nm. The particle diameter equal to or smaller than 100
nm can reduce light scattering by the silica fine particles 31 dispersed
in the resin base material 30 to a level causing largely no problem, and
the particle diameter equal to or smaller than 50 nm can reduce light
scattering to a level causing almost no problem.

[0059]The temperature refractive index variation dn/dT of the UV curable
resin 26 that does not contain the silica fine particles 31 is
-1.2×10-4(/° C.), and dn/dT of the silica fine particle
31 is 8.0×10-6(/° C.). Adding cross-linking agent after
the silica fine particles 31 are dispersed in the resin base material 30
can reduce the temperature refractive index variation of the UV-curable
resin 26.

[0060]However, it is difficult to design a diffractive optical element
having high diffraction efficiency in a wide wavelength region only by
dispersing the silica fine particles 31. Therefore in this embodiment,
fine particles (second fine particles) 32 made of ITO (second material)
are also dispersed (mixed) in the resin base material 30, which makes it
possible to adjust the refractive index and the Abbe constant of the UV
curable resin 26. As a result, the UV curable resin 26 possesses a lower
refractive index and a higher dispersion (lower Abbe constant) as
compared with the glass mold material 27.

[0061]Although the second material is not limited to ITO, it is necessary
that the second material have a higher dispersion, that is, a lower Abbe
constant as compared with the glass mold material 27.

[0062]In this embodiment, the silica fine particles 31 are dispersed in
the resin base material (RC1-C001) 30 at a volume ratio of 20% with
respect to the resin base material 30, and the ITO fine particles 32 are
dispersed therein at a volume ratio of 13.8% with respect thereto. The UV
curable resin 26 in which the silica fine particles 31 and the ITO fine
particles 32 are dispersed has a refractive index nd of 1.5588 and an
Abbe constant νd of 21.6. As a result, dn/dT of the UV curable resin
26 is -9.4×10-5(/° C.), which is smaller that that of
the resin base material (RC1-C001).

[0063]It is preferable that a mixing ratio in volume of the first fine
particles such as silica fine particles to the resin base material be
equal to or higher than 20%.

[0064]On the other hand, the glass mold material 27 is K-VC79 (nd=1.6103,
νd=57.9) described above. Although the glass mold material 27 is not
limited to K-VC79, it is preferable that the glass mold material be a low
melting point glass whose glass transition temperature is equal to or
lower than 600° C. (for example, 600° C. or less and
500° C. or more).

[0065]After a diffractive grating whose grating height is 11.4 μm is
formed by a glass molding technology, uncured UV curable resin 26 is
placed on a surface of the grating, and then the UV curable resin 26 is
irradiated with ultraviolet light to be cured. As a result, a contact
type diffraction optical element is formed.

[0066]FIG. 7 shows diffraction efficiency for the first order diffracted
light, which is designed order diffracted light, of the diffractive
optical element 1 of this embodiment at the standard temperature. The
diffraction efficiency is approximately 100% in the wavelength region
from about 450 nm to about 700 nm. FIG. 8 shows diffraction efficiency
for the zeroth order diffracted light and diffraction efficiency for the
second diffracted light of the diffractive optical element 1 of this
embodiment at the standard temperature. As is understood from FIG. 8, the
diffraction efficiency for each of the zeroth order diffracted light and
the second diffracted light is extremely low in the wavelength region
from about 450 nm to about 700 nm, which indicates an excellent
performance.

[0067]Furthermore, FIG. 9 shows diffraction efficiency for the first order
diffracted light of the diffractive optical element 1 of this embodiment
when the temperature of the diffractive optical element 1 rises from the
standard temperature by 20° C. Deterioration of the diffraction
efficiency is less than that shown FIG. 7, and the diffraction efficiency
is maintained at 99% or more. FIG. 10 shows diffraction efficiency for
the zeroth order diffracted light and diffraction efficiency for the
second diffracted light of the diffractive optical element 1 of this
embodiment when the temperature rises from the standard temperature by
20° C. The diffraction efficiency is a low value of about 0.2%,
which is a sufficient practical level.

[0068]Increasing the mixing ratio (volume ratio) of the silica fine
particles 31 can reduce the temperature refractive index variation. Since
the silica fine particle 31 is transparent, the increase of the mixing
ratio causes no reduction of transmittance. Actually, there is a report
example in which silica fine particles are dispersed in acrylic resin at
60% by weight (Industrial Material vol. 53 No. 7, 2007), which shows that
it is easy to disperse (mix) the silica fine particles in the resin
material.

[0069]FIG. 11 shows diffraction efficiency for the first order diffracted
light of the diffractive optical element 1 of this embodiment in which
the mixing ratio of the silica fine particles 31 is increased to 50% to
improve dn/dT of the UV curable resin 26 to
-5.6×10-5(/° C.) when the temperature rises from the
standard temperature by 20° C. Although the diffractive optical
element is produced by combining the resin material and the glass
material, deterioration of the diffraction efficiency is suppressed
extremely small. FIG. 12 shows diffraction efficiency for the zeroth
order diffracted light and diffraction efficiency for the second order
diffracted light of the same diffractive optical element 1 when the
temperature rises from the standard temperature by 20° C. The
diffraction efficiency for each of the zeroth order diffracted light and
the second order diffracted light at a wavelength of about 500 nm is
approximately 0.05%, and deterioration of the diffraction efficiency is
suppressed extremely small.

Embodiment 2

[0070]Next, description will be made of a diffractive optical element
which is a second embodiment (Embodiment 2) of the present invention.
This diffractive optical element is used for optical systems such as an
image taking optical system, as well as the diffractive optical element
of Embodiment 1. In general, a refractive index of a substance decreases
with rise of its temperature.

[0072]The second term of the above expression relates to the temperature
refractive index variation, which can be ignored in many cases.
Therefore, dn/dT can be approximated to -3Ct(n-1) in such cases. Since
the linear expansion coefficients of many substances are positive value,
dn/dT thereof is a negative value.

[0073]However, inorganic materials include a material having a negative
linear expansion coefficient which is resulted from its volume reduction
due to distortion of its crystal lattice with temperature rise. In such a
material, dn/dT is a positive value.

[0074]In the diffractive optical element of this embodiment, dn/dT is
suppressed by dispersing fine particles (first fine particles) made of
Niobium Oxide (Nb2O5) (first material), which is known as a
material having a negative linear expansion coefficient, in RC1-C001
which is a resin base material. Moreover, as well as the diffractive
optical element 1 of Embodiment 1, ITO fine particles are dispersed in
RC1-C001 to maintain high diffraction efficiency in a wide wavelength
region.

[0075]Specifically, the Niobium Oxide fine particles are dispersed and
mixed in RC1-C001 at a volume ratio of 20% with respect to RC1-C001, and
the ITO fine particles are dispersed and added in RC1-C001 at a volume
ratio of 10% with respect to RC1-C001. Thereby, UV curable resin having a
refractive index nd of 1.7366 and an Abbe constant νd of 18.16 is
produced. In this UV curable resin, dn/dT is improved to approximately
7.6×10-5(/° C.).

[0076]As a glass material, K-VC89 (nd=1.81004, νd=40.11) made by Sumita
Optical Glass, Inc. is used. A grating height is set to 7.9 μm. As a
result, good diffraction efficiency can be basically secured as shown in
FIG. 15.

[0077]As shown in FIG. 15, although diffraction efficiency for the first
order diffracted light in a wavelength region from about 500 nm to about
700 nm is good, diffraction efficiency at a wavelength of about 400 nm,
which is however in an unnoticeable color region, is deteriorated to an
unignorable level. Such deterioration of the diffraction efficiency can
be reduced by adding other fine particles to the resin base material so
as to fine-tune physical property values of the resin material or by
changing the glass material.

[0078]FIG. 16 shows diffraction efficiency for zeroth order diffracted
light and diffraction efficiency for second order diffracted light of the
diffractive optical element of this embodiment. The diffraction
efficiency for each order diffracted light has a good value in the
wavelength region from about 500 nm to about 700 nm.

[0079]This embodiment described the case where the fine particles made of
niobium oxide having a negative linear expansion coefficient are
dispersed in the resin base material. However, materials having a
negative linear expansion coefficient other than niobium oxide include
zirconium tungstate (ZrW2O8) and Si oxide
(Li2O--Al2O3-nSiO2). Dispersing fine particles made
of these materials in the resin base material makes dn/dT of the resin
material positive.

[0080]Moreover, recent researches reported that, in a material having
manganese nitride (Mn3XN) as a basic structure, adding germanium
(Ge) to the X part therein provides a negative linear expansion
coefficient. Dispersing fine particles made of this material in the resin
base material can suppress the temperature refractive index variation of
the resin material.

[0082]However, it is necessary to add the fine particles made of the
second material which is a material having a high dispersion to the resin
base material in order to obtain high diffraction efficiency of the
diffractive optical element over a wide wavelength region.

[0083]Further, although various materials mentioned above can be selected
from a viewpoint of the refractive index and the linear expansion
coefficient, it is necessary that attention be paid to a material having
an extremely low transmittance in some fields in which the low
transmittance material is used. Although a material having a high
transmittance such as SiO2 causes no problem, it is necessary to
consider that increasing an additive amount of the fine particles made of
the second material increases influence on the transmittance.

[0084]As described above, according to each embodiment, use of the resin
material in which the first fine particles are dispersed in the resin
base material can easily bring dn/dT of the resin material close to dn/dT
of the glass material. As a result, deterioration of the diffraction
efficiency with temperature change can be prevented. Further, dispersing
the second fine particles in the resin base material for adjusting
dispersion characteristics of the refractive index can realize a
diffractive optical element having high diffraction efficiency in a wide
wavelength region.

[0085]While the present invention has been described with reference to an
exemplary embodiment, it is to be understood that the invention is not
limited to the disclosed exemplary embodiment. The scope of the following
claims is to be accorded the broadest interpretation so as to encompass
all modifications, equivalent structures and functions.

[0086]For example, each of the above embodiments described the contact
type diffractive optical element in which the resin material and the
glass material are in close contact with each other and the diffractive
grating is formed at the close contact portion between the resin material
and the glass material. However, an alternative embodiment of the present
invention includes a multilayer diffractive optical element in which a
resin material and a glass material face each other with an air layer
therebetween and a diffractive grating is formed at a facing portion
between the resin material and the glass material.

[0087]Moreover, each of the above embodiments described the case where the
diffractive optical element is used for the image taking optical system.
However, an alternative embodiment of the present invention includes a
case where the diffractive optical element is used for optical systems
other than the image taking optical system (or for optical apparatuses
other than the image pickup apparatus).

[0088]This application claims the benefit of Japanese Patent Application
No. 2008-304460, filed on Nov. 28, 2008, which is hereby incorporated by
reference herein in its entirety.